Soft-solderable molybdenum and tungsten surfaces containing diffused tin

ABSTRACT

MOLYBDENUM AND TUNGSTEN CAN BE TINIDED IN A FUSED SALT BATH. THE SURFACES SO PRODUCED ARE READILY WET BY MOLTEN SOFT SOLDER AND, THEREFORE, SUCH SURFACES CAN BE READILY SOFT-SOLDERED BY CONVENTIONAL SOFT-SOLDERING TECHNIQUES TO THEMSELVES OR TO OTHER OBJECTS WHICH CAN BE SOFT-SOLDERED.

United States Patent US. Cl. 29-495 4 Claims ABSTRACT OF THE DISCLOSURE Molybdenum and tungsten can be tinided in a fused salt bath. The surfaces so produced are readily wet by molten soft solder and, therefore, such surfaces can be readily soft-soldered by conventional soft-soldering techniques to themselves or to other objects which can be soft-soldered.

This application is a division of my copending application, Ser. No. 835,737, filed June 23, 1969, now US. Pat. 3,567,598 which, in turn, is a continuation-in-part of my now abandoned but then copending application Ser. No. 593,324, filed Nov. 10, 1966, and assigned to the same assignee as the present invention.

This invention relates to molybdenum, tungsten, molybdenum-tungsten alloys and molybdenum-base metal and tungsten-base metal compositions containing sufiicient tin in their surfaces that they are readily wet by softsolders.

In my copending application, Ser. No. 835,737, from which this application is divided, I disclose and claim a process whereby a uniform, tough, adherent tinide coating can be formed on molybdenum, tungsten, molybdenum-tungsten alloys, molybdenum-base metal and tungsten-base metal compositions employing low current densities, that is, current densities in the range of 005-10 amperes/dmF.

In this process, tin metal is employed as the anode and is immersed in a fused salt bath composed essentially of a member of the class consisting of the alkali metal fluorides, mixtures thereof and mixtures of the alkali metal fluorides with calcium fluoride, strontium fluoride or barium fluoride and containing from 0.01-5 mole percent of tin fluoride The cathode employed is the base metal upon which the deposit is to be made. I have found that such a combination is an electric cell in which an electric current is generated when an electrical connection, which is external to the fused bath, is made between the base metal cathode and the tin anode. Under such conditions, the tin dissolves in the fused salt bath and tin ions are discharged at the surface of the base metal cathode where they form a deposit of tin which immediately diffuses into and reacts with the base metal to form a tinide coating.

A particularly novel aspect of this invention is that tungsten and molybdenum, in so far as I can determine from the literature or my own tests, can not be alloyed with tin. Despite this fact, I have found that I can diffuse tin into the surface of these two metals by my process. These tinided surfaces of molybdenum, tungsten or alloys of these two metals with each other, are readily wet by molten soft solders. Since molybdenum, tungsten or molybednum-tungsten alloys can not be soft-soldered by conventional techniques, my process provides a means by which articles fabricated from such metals or alloys can be rendered solderable so that they can be joined together or other metals joined to them with soft solders using the conventional techniques used for soft soldering, generally by use of a heated soldering iron. This procedure is applicable to all those molybdenum base metal compositions and tungsten-base metal compositions which are not readily wet by soft solder, i.e., the amount of other metal or metals alloyed with the tungsten or molybdenum, is insufficient to render the composition soft-solderable, even though these other metals themselves, may be wettable by molten soft solder.

In the specification and claims I use the term tinide" to designate any solid solution or alloys of tin with the base metal regardless of whether the base metal does or does not form an intermetallic compound with tin in definite stoichiometric proportions which can be represented by a chemical formula.

The rate of dissolution and deposition of the tin are self regulating in that the rate of deposition is equal to the rate of diffusion of the tin into the base metal cathode. The deposition rate can be decreased by inserting some resistance in the circuit. A faster rate can be obtained by impressing a limited amount of voltage into the circuit to supply additional direct current.

The alkali metal fluorides which can be used in accordance with the process of this invention include the fluorides of lithium, sodium, potassium, rubidium and cesium. However, it is preferred to employ an eutectic mixture of sodium, potassium and lithium fluorides because some free alkali metal is produced by a displacement reaction at the higher operating temperatures and rubidum and cesium are volatilized with obvious disadvantages. Because tin is chemically much less reactive in the fused salt baths than the other metals I have used for metalliding, volatilization of potassium from the bath is not the problem it is in the other systems. Mixtures of the alkali metal fluorides with calcium fluoride, strontium fluoride and barium fluoride can also be employed as a fused salt in the process of this invention.

The chemical composition of the fused salt bath is critical if good tinide coatings are to be obtained. The starting salt should be as anhydrous and as free of all impurities as is possible or should be easily dried or purified by simply heating during the fusion step. Because oxygen interferes, tiniding must be carried out in the substantial absence of oxygen. Thus, for example, the process can be carried out in an inert gas atmosphere or in a vacuum. By the term substantial absence of oxygen it is meant that neither atmospheric oxygen nor oxides of metals are present in the fused salt bath. The best results are obtained by starting with reagent grade salts and by carrying out the process under vacuum or an inert gas atmosphere, for example, in an atmosphere of nitrogen, argon, helium, neon, krypton or xenon.

It was also found that hydrogen cannot be permitted to come in contact with the salt for it reduces tin fluoride, formlng HF and tin. I have sometimes found that even commercially available reagent grade salts must be purified further in order to operate satisfactorily in my process. This purification can be readily done by utilizing scrap metal articles as the cathodes and carrying out the initial tiniding runs with or Without an additional applied voltage, thereby plating out and removing from the bath those impurities which interfere with the formation of high quality coatings.

It is advantageous to conduct the iding process in the absence of carbon, because carbon forms a very stable metal carbide on the surface of tungsten and molybdenum thereby rendering these surfaces diflicult to further tinide the base metal and giving less firmly adhering deposits. I have found that carbon can be removed from the fused salt bath by operating it as a cell employing as a cathode, the metals such as molybdenum or tungsten, until the car bide coating is no longer formed on the surface of the metal.

The form of the anode is not critic'al. Tin is used as an anode by immersing a graphite crucible filled with tin into the molten salt. Since carbon particles will interfere with diffusion of the tin into the base, such as tungsten, tightly to form coatings with tin but in addition to the formation of a metallide coating, plating of the iding agent occurs over the diffusion layer.

Very low current densities (0.0l0.1 amp./dm. are often employed when diffusion rates are correspondingly woven metal cloths, such as molybdenum, should sur- 5 round the graphite crucible. low, and when very dilute surface solutions or very thin 'In order to produce a reasonably fast plating rate and coatings are desired. Often the composition of the difto insure the diffusion of the tin into the base metal to fusion coating can be changed by varying the current form a tinide coating, '1 have found it desirable to operate density, producing under one condition a composition suitmy process at a temperature of from about 500l100 C. able for one application and under another condition a It is usually preferred to operate at temperatures of from composition suitable for another application. Generally, 600-1000 C. When metalliding tungsten and molybdehowever, current densities to form good quality tinide num metal, where diffusion is slow, I prefer to operate coatings fall between 0.5 and 5 amperes per dm. for the at temperatures of at least 700 0., and .preferably at least preferred temperature ranges of this disclosure. 900 C. If an applied is used, the source, for example,

The temperature at which the process of this invention a battery or other source of direct current, should be conis conducted determines to some extent the particular nected in series with the external circuit so that the negafused salt bath employed. For example, an eutectic of tive terminal is connected to the external circuit, terminatsodium and lithium fluorides can be employed at tempera-' ing at the metal being metallided and the positive terminal tures no lower than 675 C., because of its melting point. is connected to the external circuit terminating at the In the range from 600 C.l000 C., I can employ the metal anode. In this way, the voltages of both sources are eutectic of lithium and potassium fluorides and the eutectic algebraically additive. lithium, sodium and potassium fluorides as the fused salt. As will be readily apparent to those skilled in the art, Above 850 C., lithium fluoride can be used. A'bove 900 measuring instruments such as voltmeters, ammeters, re- C., elimination of potassium fluoride, will reduce fogging sistances, timers, etc., may be included in the external cirin the Wapor space due to volatilizing potassium fluoride. cuit to aid in the control of the process.

When an electrical circuit is formed external to the The tinided base metal can readily be wetted by soft fused salt bath by joining the tin anode to the base metal solder. For example, although molybdenum and tungsten cathode by means of a conductor, an electric, current will cannot be soldered by conventional techniques, I have unflow through the circuit without any applied electromotive expectedly found that by tiniding the surface of these base force. The anode acts by dissolving in the fused salt bath metals they can be soft soldered by using an ordinary to produce electrons and tin ions. The electrons flow soldering iron. Other uses will be readily apparent to through the external circuit formed by the conductor and those skilled in the art as well as other modifications and the tin ions migrate through the fused salt bath to the base variations of the present invention in light of the above metal cathode to be metal lized, where the electrons disteachings. charge the tin ions, forming a coating. The amount of EXAMPLE 1 current can be measured with an ammeter which enables One to readily calculat t am 0f m tal ng d An eutectic mixture of reagent grade lithium and potasposited on the basemetal cathode and being converted to ium fluorides (18 pounds) was charged into a Mono] the metallide layer. Knowing the area of the article being alloy lin (5%" diam. x 17%" deep) that wa th fit. plated, it is possible to calculate the thickness of the t d i to a teel pot (6%" diam. x 18" deep). The pot metal'lide coating formed, thereby p ng accurate was fitted with a nickel plated steel flanged lid which concontrol of the process to obtain any desired thickness of r i ed a w ter channel for cooling, two ports (2%" in the metallide layer. diameter) for glass electrode towers and two 1" ports Although the Process Operates y satisfactc'fily Withfor a thermocouple probe and a vacuum connection or out impmssing y additional electromotive force on the gas bubbler. The whole apparatus was encased in an elecelectrical circuit, 1 have found it Possible to pp y Small trical furnace for heating. A vacuum was pulled on the Voltage when it is desired to Obtain whstan'i current cell and the salt melted. Argon was let into the cell, and sities during the reaction, and to increase the deposition i h an argon fl sh to keep air out, 100 g. of stannous fate of the tin being dEPOSited Without exceeding the fluoride was added. A /2" diameter graphite crucible, fusion rate of the iding agent into the base metal cathode. fill d with 50 g, of tin and perforated with holes just The addit al h u d t eXCeed Volts and above the level of the tin was then immersed in the salt preferably should fall between 0.1 to 0.5 volt. as an anode. Using argon as an inert gas, a series of tinid- Since the diffusion rate of tin into the cathode article ing runs were made. The runs are summarized belowin varies from one material to another, with temperature, Table I.

TABLE I Percent Tem- Current coupera- Time density, Weight lomblc Run ture, (minamps/ gain, efli- No. Metal C utes) dm. 1 grams ciency. Description of coating f?lll cli fi l fii 1, (lg id riii l t, mat finish, smooth, harder than molyba Tungsten"-.- 1,000 15 1.00 0.121 45 0.3 1 350, mat finish, smooth.

and with the thickness of the coating being formed, there Strips of molybdenum and tungsten (4" x 1" x 0.20") is always a variation in the upper limits of the current tinided in accordance with the procedures set forth in densities that may be employed. Therefore, the deposition Table I were readily wetted by soft solder using either a rate of the iding agent must always be adjusted so as not soldering iron or a gas flame and conventional soft solder to exceed the diffusion rate of the iding agent into the paste fluxes. Such strips when soldered together or when substrate material if high efliciency and high quality diffusion coatings are to be obtained. The maximum current density for good tiniding is 10 amperes/dm. when operating Within the preferred temperature ranges of this disclosure. Higher current densities can sometimes soldered to other metals, for example, copper, nickel and iron made strong metal bonds. Joints, having an area of one-half square inch, would support a weight of at least 150 lbs. On being pried apart, the bonds tore in the middle be used of the solder layer rather than releasing from the molybdenum or tungsten surface. In marked contrast, when the molybdenum and tungsten strips which were not tinided, were soldered in the same way, the joints were easily separated with the bond failure being due to the solder easily releasing from the tungsten and molybdenum surfaces.

Microscopic examinations and cross-sections of the tinided samples showed that tin had diffused into the molybdenum and tungsten surfaces to a limited but significant extent, rather than being a tin coating on the surface. X-ray emission spectroscopy showed that traces of nickel often accompanied the tin when the tiniding was performed in a Monel alloy vessel but that only tin was present when the tiniding was done in high purity copper vessels. Apparently a small amount of nickel from the Monel alloy vessel had dissolved in the fused salt bath and traces of nickel had codeposited and codiffused with the tin. This is not undesirable insofar as the coating is concerned, if nickel can be tolerated or is desired to produce a tenary coating, since such surfaces can also be soft-soldered. However, erosion of the vessel will occur over a long period of use under such conditions. Where diffusion of only tin is desired, a Monel vessel whose surface in contact with the fused salt has been tinided by the process of this invention can be used in place of a copper vessel.

It will, of course, be apparent to those skilled in the art that modifications other than those set forth in the above examples can be employed in the process of this invention without departing from the scope thereof.

The tinided tungsten and molybdenum objects are particularly useful in the production of semiconductor devices. The tinided surface can be soldered to copper wire, or other soft-solderable surfaces, etc., to join such devices together with other electrical components. A tinided tungsten or molybdenum wire is also useful in electrode applications.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A metal substrate selected from the group consisting of molybdenum, tungsten, molybdenum-tungsten alloys, molybdenum-base metal compositions and tungsten-base metal compositions which are not readily Wet by molten soft solder, having a tinide surface containing sufficient tin diffused therein to render said surfaces readily Wet by molten soft solder.

2. The composition of claim 1 wherein the metal substrate is molybdenum.

3. The composition of claim 1 wherein the metal substrate is tungsten.

4. A composite structure comprising (a) a substrate having the dilfused tin surface of claim 1, (b) a softsolderable object, and (c) an interposing layer of soft solder fused to and uniting (a) to (b).

References Cited UNITED STATES PATENTS 3,368,125 2/1968 Pasierb 3l7237 3,308,353 3/1967 Gwyn -176 X 2,945,285 7/1960 Jacobs 148-177 X 2,266,330 12/1941 N'achtman 20437 T 3,518,753 7/1970 Heidenreich 3l7234 M CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

